I'm optimizing a case that is well approximated by
multiple layers of modules where every module in a layer
imports all the modules in the layer below.

It turns out I regressed performance on such cases in 7fea7121.
I'm adding a test case to track improvements and prevent
future regressions.

Test Plan: ./validate

Reviewers: simonmar, austin, bgamari

Reviewed By: simonmar

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3575